Gain control system for seismic amplifiers



Oct. 20, 1953 LOPER 2,656,422

GAIN CONTROL SYSTEM FOR SEISMIC AMPLIFIERS Filed Oct. 25, 1948 3 Sheets-Sheet l E V F119. 2 (c1) Fzlg. 3

[67 0 JZ'me George p E /1 INVENTOR. (C1) BY Fly. 4. r

ATTORNEY Oct. 20, 1953 G. B. LOPER 2,656,422

GAIN CONTROL SYSTEM FOR SEISMIC AMPLIFIERS Filed Oct. 25, 1948 Sheets-Sheet 2 I I L L 36 George B. Lo oer INVENTOR BY AW ATTORNEY G. B. LOPER Oct. 20, 1953 GAIN CONTROL SYSTEM FOR SEISMIC AMPLIFIERS Filed Oct. 25, 1948 5 Sheets-Sheet 3 DNN NNJ N ME Elm. A. QNN

www A REM mmw George B. Lgper INVENTOR.

ATTORNEY Patented Oct. 20, 1953 GAIN CONTROL SYSTEM FOR SEISBHG- AMPLIFIERS George B. Loner, Dallas, Tex., asslgnor, by mesne assignments, to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a corporation of New York Application October 25, 1948, Serial No. 56,456

5 ClaimS- (Cl. 179-171) This invention relates to seismic prospecting and more particularly to a master control system for varying the gain of the seismograph amplifiers as a predetermined function of time.

In seismic prospecting it is customary to detonate a charge of explosives at a point on or near the earth's surface and record electrical impulses produced by a geophone upon reception of seismic waves at selected points spaced from the detonation point From the data thus obtained, it is possible to determine the depth of the horizons from which the seismic waves have been reflected. Calculation of the depth of the subsurface formations involves the lapsed time from the instant of detonation of the explosive charge to the instant of arrival of reflected waves at the point of detection, the velocities of the various strata encountered in the travel path of the seismic waves, and the geometrical relationship between the points of generation and detection.

With a given spacing between the points of generation and detection, the level of the energy received following the detonation of the explosive charge varies, depending upon the elastic constants of the formations in the travel path of the waves from the point of generation to the point of reception. lhe variation in energy level as a function of time may approximate an exponential curve. In a given area the variation in level depends upon the spread configuration or spacing between the points of generation and reception of the seismic waves. If seismic detectors are positioned closely adjacent the point of generation, there is a rapid transition from high energy level direct waves and shallow reflected waves following the instant of generation to relatively low level waves reflected from deeper horizons. If the spacing is made large between the receptor and the initiation point, the level of the initial waves may not exceed the level of later reflected Waves by a factor of more than 3 or 4. For very close spacing (50 to 500 feet) the ratio may exceed 1,000. If the spread configuration is maintained constant and moved from one area to another, the attenuation of the seismic signals varies with time in a manner determined by the underlying formations. g

It is general practice to record signals detected at a plurality of points following the detonation of the explosive charge The signals from the geophones or detectors are amplified and subsequently recorded as traces or undulating lines on a photographic strip. In order to delineate or to interpret the record, it is necessary that each of the traces be recorded at a substantially constant average amplitude with the maximum thereof limited so that the adjacent traces do not overlap to such an extent that one trace may not be distinguished from the others.

Various methods and devices have been utilized to control the amplitude of the record traces. Master gain-control systems associated with the amplifiers have been utilized to vary the amplifier gain. Automatic volume control systems producing a voltage dependent upon the ampli tude of the output signal of each amplifier also have been utilized in an attempt to maintain the average trace amplitude constant. Most satisfactory results ordinarily are obtained with a system which combines automatic volume control individually responsive to the output of each amplifier, and a master control system connected to all amplifiers. In such systems the master control compensates for the general or long period variation of the signal amplitude, the automatic volume control functions to compensate for minor or short period variations.

The system of the present invention is designed to produce a master control voltage which may be made to vary with time following the initiation of the seismic waves to match the attenuation cl decay characteristic of the seismic event under widely varying spread configurations or in different areas having contrasting elastic constants. The invention, in one form thereof, comprises two condensers connected in series and in circuit with the grid-cathode impedance of one or more tubes in each of the seismic amplifying channels. A source of potential is utilized to charge each of the condensers to a predetermined maximum value. Normally non-conductive paths in circuit with each of the condensers, when rendered conductive upon detonation of the explosive, permit the accumulated charges to flow from the condensers, causing variation with time of the potential across them. The sum of the voltages across the two series condensers applied to the control grids of the amplifiers is effective in changing the gain thereof. The time constants of the discharge paths are variable. In one form of aesasaa the invention, a constant current device in the discharge path of one of the condensers causes variation in voltage linear with respect to time.

Though the circuit arrangement requires relatively few components, the amplifier gain may be chan d inversely as the average amplitude of the seismic waves decreases following detonation of the explosive charge.

For a more detailed explanation of the invention and for further obiects and advantages thereof, reference may now be had to the following description taken in coniunction with the ac companying drawin s, in which:

Fig. 1 is a schematic diagram of a master control system embodying the present invention;

Fig. 2 illustrates one control voltage function obtained with the control circuit of Fig. 1;

Fig. 3 illustrates variations of the control voltage function of Fig. 1;

Fig. i graphically illustrates a modified expander-contractor action;

Fig. 5 is a modified form of a master control system;

Fig. 6 illustrates a further control voltage function, and

Fig. '7 schematically illustrates the master control system of Fig. 1 applied to a seismograph amplifying channel.

Referring now to Fig. 1, there has been illustrated a master gain-control system which includes condensers C1 and C2. The condensers are connected in series and in circuit with the suppressor grid-cathode impedance of an amplifier tube is. The circuit includes switch ll, potentiometer l2 and conductor H! which is connected to the suppressor grid Hla of tube It. The circuit is completed through cathode 10b and a cathode biasing circuit M to ground. The sum of the voltages across condensers C1 and C2 is applied to the suppressor grid Hm of the tube [0. In order to compensate for minor variations in the gain of individual detecting-amplifying channels, a bias battery i5 is placed in circuit with the resistor l2. A portion of the bias voltage, depending upon the position of the variable tap, is added to the sum of the voltages across condensers Ci and C2 and applied to the grid I011 of the tube It.

In one form of the invention a source of potentiel 2Q connected in circuit with condenser C1 is utilized to charge condenser C1 to a voltage of predetermined magnitude. The charging circuit includes resistor 22, conductor 23, a constant current device E l, conductor 25, condenser C1, conductors 2'? and 28, ammeter 39 and battery tap 3!.

In a similar manner a source of potential All is utilize-:1 to charge condenser C2 to a predetermined voltage through a circuit which includes resistor 4 i, conductor 42, potentiometer 43, conductor condenser C2, conductors 45 and 28, ammeter 3!} and battery tap 45.

It will be observed that batteries 20 and 4-3 are connected opposite in polarity and, thus, charge the condensers C1 and C2 in opposite senses with respect to the output point 35 and the ground point 36. More particularly, condenser C1 will be charged with the grounded terminal positive and the upper terminal negative, while the condenser C2 is charged with the lower terminal negative and the upper terminal positive.

Separate discharge pads are provided for condensers C1 and C2. The discharge path for condenser C1 includes the impedance device 24 which.

in one form, may be a constant current circuit, conductor 23, gas triode or thyratron 5!, battery 52 and conductors 2S and 22'. The discharge path for condenser C2 includes conductor M, potentiometer 3, conductor :2, thyratron 53, battery 5 and conductors 28 and 55. The thyratrons 5i and 53 in the discharge paths of condenser C1 and C2, respectively, are normally non-conductive by reason of a negative bias applied to the control grids of the thyratrons. More particularly, the grid of thyratron 5! is connected in a circuit which includes transformer econdary winding 56 and bias battery 55a. The grid of thyratron 53 likewise is included in circuit with transformer secondary 57 and bias battery 51 a. As is classic in the operation of thyratrons, the tube will remain non-conductive until the grid potential is raised sufficiently to allow the thyratron to fire. Upon detonation of the exposive charge, triodes 5i and 53 are rendered conductive, allowing the condensers C1 and C2 to discharge. More particularly, a voltage pulse from a blaster (not shown) or otherwise produced in timed relation with the initiation of seismic waves, is applied through transformer 55 to the grids of thyratrons 5i and 53.

The batteries 52 and 54 are preferably chosen as to substantially equal the voltage drop across the thyratrons 5i and 53 when conducting. When such is the case the potential across, for instance, battery 52 is equal and opposite in polarity to the drop across the thyratron 5|, and the net voltage between the point and the common juncture between battery 52 and 54 will depend only upon the charge on condenser C2.

When the tube 5! is fired, condenser C1 discharges through the device 24 which, for the purposes of the following explanation, and as will hereinafter be more fully described, operates in such a manner that the flow of current from condenser C1, is constant and not an exponential function as would be the case if device "A were a fixed resistance. Thus, the variation of voltage across condenser C1 as a function of time will be substantially linear, due to the action of the constant current device In contrast, the variation of the potential across condenser C2 will be an exponential curve, the shape of which depends upon the size of the resistor 33 and the size of the condenser C2. Condenser C2 and resistor 43 are preferably variable so that the time constant of the discharge circuit for condenser C2 may be varied to suit the particular requirements. By mechanically simultaneously adjusting them, a wide range of control is provided.

To simplify the description of operation it will first be assumed that no voltage is applied to condenser C2 (battery tap 35 is positioned at the end a of the battery 38). The initial voltage E35-3s and thus the voltage across condenser C1 is determined by the position of the tap 31 on battery 2%. This voltage initially biases the suppressor grid Illa of the tube it negative with respect to ground. The value of the voltage used will depend upon the amount of initial suppression desired.

The variation of the voltage from C1 as a func-- tion of time following detonation of the explosive will be substantially linear as illustrated by the dotted line 69 of Fig. 2 which is a graph of the variation in voltage ESE-36 following detonation. By varying the capacity of the condenser C1 the curve may be modified to produce a control voltage illustrated by curves GI and :82, Fig. 2. For an intermediate value or size of C1, curve 61 5 may be taken as illustrative of the variation in 1:35-36 and for a small condenser or low value of capacitance for C1, curve 62 illustrates the operation.

For an explanation of operations of the circuit of Fig. 1, which includes features in addition to those above described, assume that the charging voltage from source 20 and the size of condenser C1 remain constant. In such case, for each op-- eration, a voltage function as illustrated by curve 60 will be obtained from condenser C1. That voltage function will then be modified by the action of condenser C2 and its circuit to produce an output control voltage E35-36 which may satisfy the requirements under varied operating conditions.

For example, if it is desired to record the first arriving energy or the first break at a high amplifier gain and thereafter abruptly to reduce the gain to record reflected energy at an optimum amplitude, the grid |a oi tube I0 will be initially at substantially ground potential. To effect such operation, the tap at will be positioned at the same potential with respect to the positive side of battery 40 as tap 3| with respect to the positive side of the battery 20, thus charging condenser 02 to the same potential and opposite in polarity with the potential of condenser C1. At the instant the explosive charge is detonated, the potential E35-36 is zero. If the time constant of CzR4a is relatively small, condenser C2 will discharge as illustrated by curve 65 of Fig. 3. The output voltage E35-3o across condensers C1 and C2 applied to the suppressor grid lfia following detonation, illustrated by solid line 66, is equal to the algebraic sum of the curve 69 and curve 65 of Fig. 3. Thus, the suppressor grid lea, initially at ground potential for maximum gain, is made negative shortly after the detonation of the explosive to reduce the gain to such a low value that the high amplitude shallow reflections may be recorded with minimized overlapping of adjacent traces. Thereafter, the gain control voltage decreases to increase the gain in compensation of the attenuation of later arriving reflections.

When it is desired to maintain the suppressor grid Illa of tube It at a relatively constant potential and negative with respect to ground for a given period following detonation of the explosive charge, battery tap 45 may be so adjusted that the algebraic sum of the effective voltages from batteries 20 and all will substantially equal the desired negative potential for the suppressor grid Hia of tube l9. The time constant of C2-R4s is adjusted to permit substantial discharge of condenser C2 by the time expansion of the amplifier gain is desired. As illustrated in Fig. 4, curve 6|), modified by the discharge curve 61 of condenser C2, effects an output or a total voltage curve 68.

The foregoing discussion and the illustrated curves obtained using the master control of the present invention indicate the wide variety of control functions which may be obtained. By varying condensers C1, C2, resistor 43 and the battery taps 3| and 46, control curves of various desired shapes may be had within wide limits.

Since, after being rendered conductive, the grid control of the thyratrons and 53 is lost, push-button switches inserted in their plate circuits may be opened to extinguish or to render non-conductive the tubes 5| and 53. Thereafter, the condensers Cl and C2 will charge to a new value preparatory to the detonation of a second or succeeding charges. The potentiometers 70, 1| and others not shown are connected through conductors l2, 13, etc., to amplifiers other than that including tube |0, efiectively to control the gain of a plurality of amplifiers from the single master control.

In Fig. 5 there is disclosed a modification of the master control circuit of Fig. l in which a mechanical relay is utilized to complete or to render conductive the discharge circuits for condensers Cl and C2. In this circuit, as in the circuit of Fig. 1, condensers C1 and C2 are in series. The voltage E65+3G is, through potentiometer l2 and conductor |3, applied to a selected gain-control grid in one of the seismograph amplifiers. Battery 20, in circuit with condenser C1, impedance device and resistor 22 charges condenser C1 to the desired control potential. Battery 40 in circuit with condenser C2, variable resistor 43 and resistor 4| charges condenser C2 to a desired potential, opposite in polarity to that on condenser C1.

Closure of an electromagnetic relay 8! provides a low impedance or short-circuit discharge path for condensers Cl and C2 in series with the impedance device 80 and variable resistor 43 respectively. More particularly, the common juncture between condensers C1 and C2 is connected through conductors 28 and 33 to the armature 84 of the relay 8|. Conductor 85, connected to the juncture of impedance device 89 and resistor 22, is terminated at contact 86 of the relay 0| which, upon closure, completes a discharge path for condenser C1. Similarly, conductor 81 connected to the juncture of resistors 4| and 43 is terminated at a second contact 88 of the relay 8|. Closure of relay 8| also closes the discharge circuit for condenser C2.

The relay energizing circuit is so designed that the closure of the discharge circuits may be delayed for a desired period of time following detonation of the explosive charge. The input terminals 90 of the input transformer 9| are preferably connected to the blaster, or are otherwise connected to a circuit which applies a voltage pulse in synchronism or in predetermined timed relation with the detonation of the explosive charge. The transformer output is applied to the grid of thyratron B2. The grid-cathode circuit of the thyratron 92 includes the control potentiometer 93 connected across the secondary of transformer 9|. The adjustable tap of potentiometer 93 is connected directly to the grid of the tube grid circuit further includes a resistor 94 in series with bias battery 95 and the cathode or. the tube 92. The biasing battery .35 ser' es t e function of maintaining the thyratron 83 non-conductive. An impulse of sufficient magnitude applied to the input 90 of the transformer 9| will drive the grid potential in a positive direction sufiicient to fire the thyratron 92.

The plate circuit of the thyratron includes a switch 98, a load resistor 99, the actuating coil |00 of they relay 8|, and a supply battery l0l. A condenser I03 is connected in parallel with the winding or actuating coil Hill of the relay 8|. By providing the condenser )3 of variable capacity, the time at which the relay 3| closes the discharge circuits of condensers C1 and C3 following the firing of the thyratron 92 may be controlled. If the condenser I93 and the coil I00 are properly chosen, delays in the range of from .1 to .5 second or more may be introduced to delay closure of relay 8|. Thus, the time delay Ta, Fig. 6, may be varied at the will of an operator. By providing such delay, a contraction-expansion action such as illustrated in Fig. 6 may be obtained.

The curve I06 illustrates the variation of the voltage across condenser C1 as a function of time following the detonation of the explosive charge. The curve I91, obtained with a short time constant for the circuit including resistor 43 and condenser 02, illustrates the variation of the voltage across condenser C2 following the detonation. The algebraic sum of the two voltages (curves I06 and It?) represents the control voltage Est-36 applied through conductor I3 to a suppressor grid of one of the amplifier tubes. The effective control curve is illustrated as the unbroken line or curve Hi8.

It is evident that the control voltage function illustrated in Fig. 6 may be modified in a manner similar to the modifications of the curve I50 of Fig. 2, as illustrated in Figs 2-4. The addition of the time delay provides an expander-contractor action where a time delay of more than a few tenths of a second may be desired, and may be used in instances where the modified contractor-expander action illustrated in Fig. 3 is not adequate.

In Fig. 7 the master control circuit of Fig. 1 has been illustrated as applied to a seismograph amplifying channel. Parts corresponding with those in Fig. l have been given the same reference characters.

The amplifying channel comprises three stages including tubes I0, IE8 and III. the tubes are of the pentode type, each having a control grid, screen grid and pentode grid respectively spaced between cathode and plate thereof. Each of the stages includes a grid resistor H2, cathode biasing circuit II3, plate resistor H4. The screen grids are interconnected and supplied from a source of potential +Sc. The plates of the tubes likewise are supplied from a source of 13+ potential. Cu-

pling condensers IIS are provided intermediate each stage.

A seismic detector or geophone I29 positioned on the ground and spaced from the point of detonation of the explosive charge generates electrical impulses in sympathy with seismic waves. The geophone is connected to an input transformer H9, the secondary of which is connected to the control grid of tube Hi. The signal is amplified at tube I6, subsequently at tubes H0 and III. The output of tube IiI is connected through condenser I2I to an output transformer I22 which, in turn, is connected to a multi-element recorder I23. As is well understood, in a given seismic surveying operation there will be a plurality of geophones such as geophone I20, a corresponding number of amplifying channels such as the channel comprising tubes I I Ill! and I I I, each of the channels being connected to the multi-element recorder I23. lhe recorder produces an oscillographic record having a plurality of traces, each representative of the seis mic waves received at the respective detecting stations.

Further, and as above indicated, for the best results automatic volume control is used in conjunction with the master control. The A. V. C. unit 525 is coupled to the output of the tube III throug condenser 21. The output of the A. V. C. circuit I25 is then connected to the suppressor grids of tubes I II! and I I I. The A. V. C. voltage applied to tubes [I0 and III may be derived through a circuit such as disclosed in Patent 2,306,991 to Groenendyke. The response or the time constant of the A. V. C. circuit is preferably in the order of one-tenth of a second, in which As illustrated,

anda

case minor variations in the average level or the signal impinging the geophone I20 will be compensated for.

Further, and in accordance with the present invention, the output of the master control circuit is applied to the suppressor grid of the first tube, tube III, of the amplifier channel. As illustrated, conductor I3, connected to potentiometer I2, applies the master control voltage Ext-3s to the suppressor grid Illa of tube III. With such circuit arrangement the gain of the amplifying channel may be made to vary as any of the curves illustrated in Figs. 2-4. If the master control of Fig. 5 is utilized, any of the control curves of Figs. 2-4 and 6 may be obtained. The primary of the transformer 55, Fig. 7, is connected to the output of the amplifier, one terminal of the primary 58 being connected to the plate of tube II I through coupling condenser I30. The other terminal of the primary 58 of transformer 55 is connected through conductor IBI to the ground point 36.

The device lid for maintaining the current from condenser C1 constant following energization of thyratron 5i has been illustrated in detail in Fig. *2, and comprises a pentode I60, the plate-cathode resistance of which is in series with condenser C1 and thyratron 5 I. More particularly, the cathode IE5 is connected to the plate of thyratron 5| and the plate 562 is connected to the grounded terminal of condenser C1. The suppressor grid IE3 is at cathode potential and the control grid IE4 is maintained negative by battery I55. Battery I66 connected between the screen grid It! and cathode it! is chosen so that the current from condenser C will be essentially constant. By properly choosing the operating point for the tube ISII the flat E -I characteristic curve of the pentode may be utilized to maintain a constant current how of, for example, approximately microamperes from condenser C1 over a wide range of plate voltages. Such use of the flat Ep-Ip curve means that as the voltage across condenser C1 decreases the impedance of device 59 also decreases and by an amount which keeps constant the discharge current of condenser C1. Switches 5 Ia and 53a in the plate circuits of thyratrons 5| and 53, respectively, are mechanically coupled to the switch IE5 in a line comprising conductors lfisa and I682). When switch I68 is closed the constant current device 50 is lay-passed. In operation, with the condensers C1 and C2 charged to a desired voltage, switches 5Ia, and 53a closed and switch IE8 open, energization of the thyratrons 5i and 53 causes discharge of the condensers C1 and C2 through their respective discharge circuits. When the recording period has been completed, actuation of the switches 5m, 53d and I E8 simultaneously renders the triodes 5i and 53 non-conductive and closes the path in shunt with the constant current device 58 to permit condenser C1 to be recharged.

-ssuming, Fig. '1, that the charging potentials on condensers C1 and C2 are equal and opposite, as illustrated in Fig. 3, the first impulses or first break energy impinge upon the geophone I20, pass through the amplifying channel, and are recorded on one of the galvanometer traces of recorder I23 with the amplifier at full gain. Upon passage or" the first impulse through the plate circuit of the tube ill the impulse is applied by way of condenser I 30 to the master control circuit, firing the thyratrons 5i and thus initiating the master control action which, upon the rapid discharge of condenser C2, such as illusg trated'by curve 65 of Fig. 3 or curve I01 of Fig. 6, effects rapid application of a high negative suppressor grid voltage abruptly to reduce the amplifier gain to the desired level. Thereafter, the gain of the amplifier is dependent upon the variation of the voltage across condenser C1.

By way of explanation, and in no way intended as a limitation, a circuit having the following components and circuit constants may be taken as typical. The thyratrons 5| and 53 may be of the type 2050. The cathode plate drop of this thyratron is approximately 8 volts when conductive. Accordingly, and in accordance with the present invention, batteries 52 and 54 may con veniently be 7% volts. In the application to which the control circuit was utilized the tube it was a pentode such as a 6J7. The batteries and were chosen to ermit application of voltages to the suppressor grid of tube 10 sufiicient to cause operation near plate current cut-off. In one embodiment of the circuit the batteries 28 and 40 were 22 /2 volts. The condensers C1, C2, and resistor R43, and the constants of the constant current device may be chosen in accordance with well-knovvn principles to produce discharge of the condensers in times suitable for specific applications. Generally speaking, the time constant of the circuit comprising condenser C1 and device 50 would be of the order of two seconds for a reflection seismograph, while the time constant of the circuit including condenser C2 and resistor R43 would be variable between 0 and 1 second. Though the constant cur.w rent device 50 has been included in the description of the invention, a fixed resistance may be utilized in place thereof, if it is desired to have the long-time expansion curve (curve 69, Figs. 2-4) exponential in shape.

Though the invention has been illustrated by several modifications thereof, it is understood that further modifications within the scope of the appended claims may now suggest themselves to those skilled in the art.

What is claimed is:

1. A gain control for an amplifier for seismic prospecting where the average amplitude of seismic waves received at a recording station decreases as a function of time, comprising a gain controlling circuit for said amplifier having therein a pair of series-connected condensers, a

for each of said condensers, one of said discharge v circuits including means for maintaining uniform the value of the discharge current, and means operable in timed relation with the initiation of the seismic waves for completing said discharge circuits for controlled variation of the sum of the voltages of said condensers for varying the gain of said amplifier.

2. In seismic prospecting where the average amplitude of seismic waves received at a recording system decreases as a function of time, the combination of a seismic amplifier having a, gaincontrolling circuit, a pair of condensers associated with said gain-controlling circuit of said amplifier to compensate for said decrease in amplitude, means for charging said condensers to voltages of predetermined magnitude of opposite polarity, a circuit applying the algebraic sum of said voltages to said gain-controlling circuit, means including a discharge circuit and an impedance device whose impedance decreases with a decreasing voltage applied to it connected in said discharge circuit for discharging one of said condensers uniformly with respect to time, and means including a second discharge circuit and a constant impedance device for discharging the other of said condensers exponentially with respect to time following the initiation of said seismic waves to vary the gain of said amplifier during reception of said waves proportional to the instantaneous sum of said voltages.

3. A system including a vacuum tube amplifier for seismic signals and a control circuit for varywith time the gain of said amplifier, comprising a condenser having one terminal connected to a grid of said amplifier, a second condenser having one terminal connected to a cathode of said amplifier, the remaining terminals of said condensers being connected together, circuit means including sources of potential independently to charge said condensers to predetermined voltages of opposite polarity in said control circuit, a normally non-conductive discharge circuit for each of said condensers, means responsive to the signal output of said amplifier for rendering both of said discharge paths conductive, a constant current device in said discharge circuit with one of said condensers for producing linear discharge thereof with respect to time, and resistance means in said discharge c cuit of the other of said condensers for producing exponential discharge thereof with respect to time, the gain of said amplifier varying with change in the algebraic sum of the voltages across said condensers.

4. A system including a vacuum tube amplifier for seismic signals and a control circuit for varying with time the gain of said amplifier, comprising a condenser having one terminal connected to a grid of said amplifier, a second condenser having one terminal connected to a cath- Ode of said amplifier, the remaining terminals of said condensers being connected together, circuit means including sources of potential independently to charge said condensers to predetermined voltages of opposite polarity in said control circuit, a normally non-conductive discharge circuit for each of said condensers, means actuated in timed relation with the initiation of seismic waves for rendering both of said discharge paths conductive, a constant current device in said discharge circuit with one of said condensers for producing linear discharge thereof with respect to time, and resistance means in said discharge circuit of the other of said condensers for producing exponential discharge thereof with respect to time, the gain of said amplifier varying with change in the algebraic sum of the voltages across said condensers.

5. In seismic prospecting where the average amplitude of the seismic waves received at a re cording system decreases as a function of time, the combination of a seismic amplifier having a gain-controlling circuit, a pair of condensers associated with said gain-controlling circuit of said amplifier, a separate direct-current source of supply for each of said condensers, charging circuits each including one of said sources of supply for charging said condensers to voltages of predetermined magnitude of opposite polarity, said condensers being associated with said gaincontrolling circuit for application thereto of the algebraic sum of said voltages for controlling the gain of said amplifier, a normally non-conductive discharge circuit for each of said condensers, means actuated in timed relation with the ini References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Brillouin Mar. 25, 1930 Stansbury Dec. 17, 1935 Bull Sept. 10, 1940 Weber July 29, 1941 Minton July 25, 1944 Cloud May 8, 1945 Boucke July 16, 1946 Petty Oct. 29, 1946 

